The present invention relates a a fluorescent display power supply device of a microwave oven.
The mechanisms of microwave ovens have become noticeably sophisticated in recent years. Reflecting this, a variety of data display devices are provided for them. Actually, many of these mechanisms use a fluorescent display tube containing a large number of display positions. A fluorescent display tube typically contains an anode which is a display unit itself, a heater emitting electrons to said anode, and grid electrodes controlling electrons. Specifically, such a multi-display-position fluorescent display tube has such a configuration in which a heater is provided in order to commonly cover all display positions, while external terminals extend from the right and the left. As a result, if the heater voltage significantly drops to a critical level from a specific voltage existing between the heater and anode, a difference in luminance will occur in the luminance between the uppermost and the lowest display positions.
A microwave oven performs a cooking by properly controlling the ON-OFF operations of either the microwave heating via magnetron or the radiation heating via the heater in accordance with the instructions of the built-in microcomputer, thus consuming large amounts of power during the heating process. This also causes the output voltage from the power transformer to vary when turning the power ON and OFF for heating operations, thus causing the luminance of the display tube to vary.
A typical circuit diagram of a conventional microwave oven is shown in FIG. 1, except for the power circuit driving the heating device. The commercial AC voltage is first transformed by the power transformer 1, followed by rectifying it into a DC voltage via a rectifying circuit 2 comprising full-wave rectifying diodes D1 through D4 and a capacitor C1, said DC voltage being converted into 100 KHz of high frequency power via an oscillation circuit 3. A secondary coil of high frequency transformer 4 is provided with terminal A for connection to the microcomputer 5, terminal B for the heater potential operating the fluorescent display tube, and terminal C for the cut-off bias of the fluorescent display tube. In addition, a secondary coil for the heater that operates the fluorescent display tube is provided. A DC voltage VDD is generated by a rectifying circuit comprising diode D5 and capacitor C5, which is provided to the microcomputer 5. A mid-range potential VH is then generated by a rectifying circuit comprising diode D6 and capacitor C6 for delivery to the heater, and said potential VH is provided the mid-point of the heater coil, thus causing the display erase potential VP to be generated in the rectifying circuit comprising diode D7 and capacitor C7. The potential VP is then sent to both the anode and grid electrodes of the fluorescent display tube 6 via resistors R4 and R5. The anode electrode of the segment of the fluorescent display tube 6 and the grid electrodes of each display position are respectively connected to the output pins of the microcomputer 5, while each of these electrodes is provided with a ground level according to the contents to be displayed. Potentials thus obtained are shown in FIG. 2, in which, VDD corresponds to -15 V, VH -24 V, and VP -28 V against the ground level VSS, respectively. Difference Ek between the lowest potential of the heater voltage (AC) and the display erase voltage VP is used for the cut-off bias voltage.
As described above, since conventional fluorescent display power supply devices drive heaters by means of high frequencies, any problem related to the difference of the display luminance can be solved. Nevertheless, they still contain complex circuit constructions, in particular, such a high frequency power oscillation circuit adversely affects broadcast receiving equipment. In addition, those conventional circuits need a large number of coil terminals for the power transformer in order to generate the cut-off bias voltage Ek.